Currently, the detection and measurement of many analytes at trace levels within a water sample requires complex laboratory equipment and a skilled technician. A variety of laboratory techniques exist. The EPA provides a list of available and approved techniques for compounds of concern. The American Public Health Association, American Water Works Association, and Water Environmental Federation publish Standard Methods for the Examination of Water and Wastewater—an extensive treatise on water analysis methodology. Although a variety of analytical detection methods can be used in water analysis, most trace-level analysis falls upon widely-used and popular, yet complex and expensive, mass spectrometry methods.
Raman spectroscopy is a potential alternative for trace analyte detection. Raman spectroscopy provides a chemical signature for a compound (in the form of a unique configuration of peaks in the reflected spectrum), but the Raman signal is typically too weak for part-per-billion detection. However, the Raman signal can be enhanced by the presence of an active surface or marker (typically a metal), creating surface-enhanced Raman spectroscopy (SERS). With an adequate interaction between the analyte and the surface, signal enhancement creates opportunities to detect very small concentrations. Some compounds, such as pyridine, naturally interact strongly with gold and silver surfaces. In some other cases a binding compound may bring the analyte and metal molecules into sufficiently close contact. For example, treatments with octadecylthiol have been used successfully for SERS on planar substrates with some analytes.
In some SERS techniques, the surface is provided by metal nanoparticles. When a metallic nanoparticle smaller than the wavelength of light is introduced into the sample, the illuminating electric field will create surface resonances if there are free electrons in the nanoparticle. The nanoparticles can be gold, silver, or copper beads, for example. These oscillating charges create an enhanced local electric field along certain directions, which results in a much stronger Raman response. “Hot spot” regions can be created where the SERS signal is greatly enhanced. These regions are most likely due to nanoparticle alignments or aggregates that create even larger electric field enhancements.
U.S. Pat. No. 8,070,956, Hand-Held Microfluidic Testing Device, describes a testing device with a cartridge receiving port for receiving a cartridge. An optical detection system in the housing is disposed to analyze a sample in channel of the cartridge. In some embodiments, markers such as gold nanoparticles are present in the channel and the optical detection system is a Raman spectroscopy system. In some embodiments, the channel includes narrow sections along a microfluidic separation channel that trap gold nanoparticles at a high density. This encourages the creation of “hot spots” through nanoparticle density at a predetermined detection location.
Although some researchers have achieved remarkable detection limits using SERS, the use of SERS to measure the concentration of a trace analyte is less developed. In general, Raman signal strength is proportional to the amount of analyte per unit area. However, one issue is that the generation of random nanoparticle hot spots leads to random signal enhancements, which interferes with correlating signal strength to analyte concentration. Another issue is that aqueous sample conditions other than analyte concentration, for example pH, can alter the analyte signal strength.